That depends . . . don't you hate that?
If you pitch a vial into a couple quarts of 1.040 starter, at about 70 degrees, and you are allowing for significant air interchange . . . a couple days will do it. You can turn off the stirplate to see if fermentation has stopped.
I suggest that you get an aquarium air pump and an in-line filter to keep the air moving (the tube does not have to be placed in the liquid). You'll get a faster, larger growth. And, because of that, you're going to need some anti-foam to keep all that yeast in your flask - but, it works GREAT.
Good luck.